1. Field of the Invention
The embodiments discussed herein relate to a method of manufacturing a silicon carbide semiconductor device.
2. Description of the Related Art
Known power semiconductor devices include silicon (Si) diodes and insulated gate bipolar transistors (IGBTs) having breakdown voltages of 400 V, 600 V, 1200 V, 1700 V, 3300 V, or higher. These semiconductor devices are each used in power converting equipment such as converters and inverters. Excellent electric properties such as a low loss, high efficiency, and a high resistance to breakdown, and a low cost are demanded of power semiconductor devices.
The following method has been proposed as a method of manufacturing a silicon carbide semiconductor device. Front surface element structures are formed such as diffusion regions and a MOS gate (an insulated gate including a metal—an oxide film—a semiconductor) structure, on a front surface side of an n−-type semiconductor substrate forming an n−-type drift layer. The n−-type semiconductor substrate is ground from a back surface side to reduce the thickness thereof to a position corresponding to the thickness of the product. Protons are implanted from the back surface after the grinding of the n−-type semiconductor substrate and the substrate is thereafter thermally treated, whereby donors are produced based on compound defects including hydrogen (H) atoms implanted into the n−-type semiconductor substrate, plural point defects in the n−-type semiconductor substrate, and the like to form the n-type diffusion layer. The n-type diffusion layer whose doping concentration is higher than that of the n−-type semiconductor substrate is an n-type field stop (FS) layer. The donors based on the compound defects including hydrogen atoms are called “hydrogen induced donors”.
Recently, the development of a semiconductor device using a silicon carbide (SiC) semiconductor having a better figure of merit (FOM) than that of a silicon semiconductor (hereinafter, referred to as “silicon carbide semiconductor device”) is actively pursued. When the rated voltage is particularly set to be a voltage equal to or higher than 10 kV, for a semiconductor device using a silicon semiconductor (hereinafter, referred to as “silicon semiconductor device”), the thickness of the n−-type drift layer influencing the maintenance of the breakdown voltage and the conduction property, has to be close to 1000 μm, and high speed operation thereof is limited. In contrast, for a silicon carbide semiconductor device, the thickness of the n−-type drift layer may be reduced up to about 100 μm. For use with voltage equal to or higher than 10 kV (for example, generation and delivery of a high voltage DC or the like), the manufacture (production) of a silicon carbide semiconductor device is extremely effective. When the rated voltage is set to be equal to or higher than 6 kV, the silicon carbide semiconductor device has to execute bipolar operation (in which both of electrons and holes are involved as charge carriers). With such high rated voltages, improvement of the doping concentration based on the n-type field stop layer is necessary also in silicon carbide semiconductor devices similar to silicon semiconductor devices, from the viewpoint of a low loss and suppression of oscillation of the current and voltage waveforms.
As a method of forming the n-type field stop layer using proton implantation, a technique has been proposed concerning degradation of the carrier (electrons and holes) mobility in the proton-implanted region (see, for example, US Patent Application 2005/0116249). As a method of forming the n-type field stop layer, conditions for thermal treatment are disclosed to recover the crystal defects generated during the proton implantation, for the thermal treatment executed after the proton implantation (see, for example, US Patent Application 2006/0286753). According to a proposed method of manufacturing an IGBT including an n-type field stop layer, the n-type field stop layer is formed using proton implantation and annealing (thermal treatment) and a collector layer is thereafter formed using ion implantation and laser annealing (see, for example, Japanese Laid-Open Patent Publication No. 2001-160559). In Japanese Laid-Open Patent Publication No. 2001-160559, the doping concentration of protons is recovered by annealing executed after the proton implantation.
According to another proposed method of forming the n-type field stop layer, the doping concentration of the protons is increased by recovering the defects by locally heating the semiconductor substrate at a temperature that is low to the extent that no outward diffusion of the protons occurs, using an electron beam or a laser after the proton implantation and before annealing to convert the protons into donors (hereinafter, referred to as “proton annealing”) (see, for example, Japanese Laid-Open Patent Publication No. 2009-99705). According to a further proposed method, oxygen (O) atoms are introduced in advance into a silicon substrate; protons are implanted from the front surface of the silicon substrate; the proton annealing is thereafter executed in a hydrogen atmosphere; the silicon substrate is thereafter ground from the back surface side thereof to reduce the thickness thereof; phosphorus (P) is ion-implanted into the back surface after the grinding; and annealing is thereafter executed using a YAG laser (see, for example, Re-Publication of PCT International Publication No. 2007-55352). In Re-Publication of PCT International Publication No. 2007-55352, the degradation of the carrier mobility in the proton-implanted region is suppressed by introducing oxygen into the silicon substrate.
According to another method, after protons are implanted from the back surface of a substrate, annealing is executed with respect to the protons by applying a YAG laser light beam and a continuous wave (CW) laser light beam from the back surface of the substrate, whereby an n-type field stop layer (an n-type diffusion layer formed by donor production of the protons) is formed (see, for example, Japanese Laid-Open Patent Publication No. 2009-176892). According to further proposed method, at least one n-type intermediate layer that includes, as a pair, two layers of an n-type field stop layer whose doping concentration is higher than that of an n−-type drift layer, and an n-type disorder reduction region whose doping concentration is lower than that of the n-type field stop layer and whose doping concentration is equal to or higher than that of the n−-type drift layer, is formed between the n−-type drift layer and a p-type collector layer (see, for example, PCT Publication WO 2013/108911). Concerning a method of forming the n-type field stop layer, it has been disclosed that protons are implanted into a silicon substrate and proton annealing is executed to thereafter further execute laser annealing and that the silicon substrate may be replaced with a silicon carbide substrate (see, for example, US Patent Application No. 2014/0151858). A method of improving the adhesion between a silicon carbide substrate and a back surface electrode has been proposed (see, for example, Japanese Laid-Open Patent Publication No. 2012-248729).